Readily bondable polyester film for optical use and laminated polyester film for optical use

ABSTRACT

A readily-adhesive polyester film for optical applications that suppresses rainbow reflections under fluorescent lamps and has excellent adhesion to a hard coating layer and exhibits excellent adhesion at high temperatures and high humidities (resistance to humidity and heat) includes a biaxially-stretched polyester film and a coating layer that is stacked on at least one side of the polyester film and produced by applying to at least one side of the polyester film an aqueous coating liquid containing a resin composition including (A) an aqueous polyester resin and (B) at least one compound that is a water-soluble titanium chelate compound, a water-soluble titanium acylate compound, a water-soluble zirconium chelate compound or a water-soluble zirconium acylate compound as main components. The mixing ratio (A)/(B) is from 10/90 to 95/5 by weight. After that the coating is dried and then stretched in at least one direction.

TECHNICAL FIELD

The invention relates to a readily-adhesive polyester film for opticalapplications that is useful as a base material for antireflection filmswhich are attached to the front side of a display screen of a toughpanel, a liquid crystal display (LCD), a cathode-ray-tube (CRT) for atelevision set or computer, a plasma display (PDP), or the like toimpart antireflection properties such that the reflection of externallight, glare, rainbow reflections, and the like can be suppressed. Theinvention also relates to a laminated polyester film for opticalapplications that includes the above film and a hard coating layerstacked on a coating layer on at least one side of the above film.

BACKGROUND ART

The front side of a display of a touch panel, a computer, a televisionset, a liquid crystal display, or the like or the front side of adecoration or the like is provided with a hard-coated film that includesa base material of a transparent plastic film and a hard coating layerthat is stacked on the base material and made from an electron beam-,ultraviolet light- or heat-curable resin. The base material of thetransparent plastic film is generally a transparent biaxially-orientedpolyester film, and in many cases, a readily-adhesive layer is providedas an intermediate layer to improve the adhesion between the basematerial of the polyester film and the hard coating layer.

The hard-coated film is required to have durability to temperature,humidity and light, and to have transparency, chemical resistance,scratch resistance, anti-fouling properties, and the like. Thehard-coated film, which is used for displays, decorations or the like,is also required to have excellent visibility and a certain designfeature. For this purpose, a certain antireflection layer with amultilayer structure of alternately stacked high and low refractiveindex layers is generally formed on the upper layer of the hard coatinglayer in order to suppress glare, rainbow reflections or the like causedby reflected light viewed from any angle.

In applications such as displays and decorations, however, there hasrecently been a demand for larger screens (larger areas) and highergrades, which is particularly accompanied by an increase in therequirement level for suppression of rainbow reflections (interferencefringes) especially under fluorescent lamps. Since most fluorescentlamps are of three-wavelength type for reproduction of daylight color,interference fringes can easily occur. There is also an increasingdemand for cost reduction by simplifying the antireflection layer. Thus,it has been demanded that interference fringes be reduced as much aspossible only by means of the hard-coated film.

It is believed that in the hard-coated film, rainbow reflections(interference fringes) can be caused by a large difference between therefractive indexes of a base material of a polyester film (for example,1.62 for PET) and a hard coating layer (for example, 1.49 for acrylicresin).

For the purpose of prevention of interference fringes by a reduction inthe refractive index difference, for example, JP H07-151902(A) disclosesa method in which the refractive index of a hard coating layer is raisedby adding metal oxide fine particles to the hard coating layer. Theaddition of metal oxide fine particles to the hard coating layer,however, leads to a decrease in the essential function of the hardcoating layer, such as transparency, chemical resistance, scratchresistance, anti-fouling properties, or the like. When an antireflectionlayer is further formed on such a hard coating layer, the antireflectionlayer needs to be optimized depending on the change in the refractiveindex of the hard coating layer.

JP 2001-71439(A) discloses another method for suppressing theinterference fringes in the hard coating layer, which focuses on localvariations in the thickness of a film and includes the steps ofproducing a readily-adhesive film followed by calendering the film toreduce the local variations in the thickness of the film. In thismethod, however, the film alone is evaluated for interference fringes,and no examination is conducted regarding interference fringes based onthe difference in refractive index at the interface formed by stackingthe hard coating layer. This method also has an increased number ofprocesses and thus has a problem with productivity.

JP 2002-241527(A) discloses an invention that focuses on unevenness inthe thickness of the layers forming a hard-coated film and defines aninterference fringe area ratio. However, the specification of thisapplication includes no specific description on the degree of theunevenness in thickness or how to reduce the unevenness in thickness. Inorder to reduce the unevenness in the thickness of each layer, forexample, the thickness of each layer should be strictly controlled sothat there should be a problem with productivity or yield.

In addition, JP 2002-210906(A) discloses a method that focuses on thebackside reflectance of a film itself and includes the steps ofcontrolling the backside reflectance to a low level and stacking a hardcoating with a specific hardness. The method disclosed in thispublication, however, essentially includes the steps of forming acoating layer with a specific refractive index and a specific thicknesson the side opposite to the hard coating layer of the hard-coated filmand controlling the backside reflectance to 0.1% or less. Thus, even thebackside must be designed when the film is designed. In addition, thebackside reflectance should be controlled to be always 0.1% or less inthe film manufacturing process by measuring the backside reflectance andoptionally by changing the conditions in cases where the backsidereflectance is out of the range, and such control of the backsidereflectance complicates the process.

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

It is an object of the invention to provide a readily-adhesive polyesterfilm for optical applications that suppresses rainbow reflections underfluorescent lamps and has excellent adhesion to a hard coating layer andexhibits excellent adhesion at high temperatures and high humidities(resistance to humidity and heat) and to provide a laminated polyesterfilm for optical applications that includes the above film and aspecific hard coating layer stacked on the film.

Means for Solving the Problems

The inventors have focused on the refractive index of thereadily-adhesive layer and controlled the refractive index of thereadily-adhesive layer by selecting the type and content of the resinand additive components for the readily-adhesive layer in such a mannerthat the refractive index differences between the polyester film baseand the readily-adhesive layer and between the readily-adhesive layerand the hard coating layer can be reduced, respectively, and thus havefound that rainbow reflections can be suppressed under fluorescent lampswhile the adhesion to the hard coating layer and the adhesion at hightemperatures and high humidities (resistance to humidity and heat) canbe maintained.

Thus, the invention is directed to a readily-adhesive polyester film foroptical applications that includes a biaxially-stretched polyester film;and a coating layer that is stacked on at least one side of thepolyester film and produced by a process including: applying, to atleast one side of the polyester film, an aqueous coating liquidcontaining a resin composition including (A) an aqueous polyester resinand (B) at least one of a water-soluble titanium chelate compound, awater-soluble titanium acylate compound, a water-soluble zirconiumchelate compound, or a water-soluble zirconium acylate compound, as maincomponents, wherein the mixing ratio (A)/(B) is from 10/90 to 95/5 bymass; drying the coating; and then stretching the coating in at leastone direction. The invention is also directed to a laminated polyesterfilm for optical applications that includes: the above readily-adhesivepolyester film; and a hard coating layer that is stacked on the coatinglayer on at least one side of the readily-adhesive polyester film andincludes an electron beam-cured or ultraviolet light-cured acrylic resinor a heat-cured siloxane resin.

EFFECTS OF THE INVENTION

When a hard coating layer is laminated on the readily-adhesive layer ofthe optical readily-adhesive polyester film of the invention, the filmof the invention demonstrates excellent antireflective properties tosuppress the reflection of external light, glare, rainbow reflections,and the like and exhibits excellent adhesion to the hard coating layerand exhibits excellent adhesion at high temperatures and high humidities(resistance to humidity and heat).

BEST MODE FOR CARRYING OUT THE INVENTION

Base Film

In the invention, the biaxially-stretched polyester film for use as abase material comprises a polyester resin and may be mainly composed ofat least one of polyethylene terephthalate, polypropylene terephthalate,polybutylene terephthalate, and polyethylene naphthalate. Among thesepolyester resins, polyethylene terephthalate is most preferred in termsof the balance between the physical properties and cost. When biaxiallystretched, polyester films can have improved resistance to chemicals orheat, improved mechanical strength or the like.

The biaxially-stretched polyester film may be of a monolayer ormultilayer type. In each layer, if necessary, the polyester resin maycontain any of a variety of additives. Examples of such additivesinclude antioxidants, light-resisting agents, antigelling agents,organic lubricants, antistatic agents, ultraviolet absorbing agents, andsurfactants.

In order to improve the handling properties of films such as slidingproperties, winding properties and anti-blocking properties or improvewearing properties such as wear-resistant properties and anti-scratchproperties, inert particles are generally added to polyester film basematerials. However, the film of the invention is for use as a base filmfor optical components and thus required to have excellent handlingproperties while maintaining high transparency. More specifically, withrespect to the transparency, the readily-adhesive polyester film for useas an optical component preferably has a total light transmittance of atleast 85%, particularly preferably of at least 90%. The higher the totallight transmittance, the better the transparency (100% is ideal). As thetotal light transmittance increases, however, the handling propertiescan decrease such that industrial-scale production could be difficult.Thus, the total light transmittance preferably has an upper limit of96%.

Therefore, the content of the inert particles in the base film shouldpreferably be as low as possible. For this purpose, it is preferred thatthe film should have a multilayer structure in which only the outerlayer contains the particles or that the film should be substantiallyfree of particles while only the coating layer contains fine particles.

Particularly, with respect to transparency, if the polyester film issubstantially free of inert particles, it should be important to improvethe handling properties of the film by adding inorganic and/orheat-resistant polymer particles to the aqueous coating liquid andforming irregularities on the surface of the coating layer. As usedherein, the phrase “substantially free of inert particles” means thatfor example, in reference to inorganic particles, the content of theparticles is less than the detection limit when the element(s) derivedfrom the particles is quantitatively analyzed by fluorescent X-rayanalysis.

In the readily-adhesive polyester film of the invention, the coatinglayer is made from a resin composition that includes (A) an aqueouspolyester resin and (B) at least one of a water-soluble titanium chelatecompound, a water-soluble titanium acylate compound, a water-solublezirconium chelate compound, or a water-soluble zirconium acylatecompound, as main components, wherein the mixing ratio (A)/(B) is from10/90 to 95/5 by mass.

When heated in the process of stretching the base film, the resincomposition produces a uniform film by the crosslinking reaction of (B)at least one of the titanium chelate compound, the titanium acylatecompound, the zirconium chelate compound, or the zirconium acylatecompound with the polyester resin. Namely, the metal chelate or acylatecompound is decomposed by the heat treatment. In the coating layer,therefore, the metal chelate or acylate compound is not present in thestate at the time when it is added to the coating liquid.

Thus, the content of the metal chelate or acylate compound in thecoating liquid may be calculated from the content of the metallicelement (Ti or Zr) in the coating layer after the heat treatment asfollows.

(1) First, the type of the chelate or acylate that has been added to thecoating liquid is identified by the residue of the chelate or acylate inthe coating layer.

(2) The content of the metal chelate or acylate compound in the coatingliquid is then calculated from the content of the metallic element (Tior Zr) in the coating layer.

The refractive index of the coating layer can be made higher than thatof the polyester resin (A) alone by increasing the composition ratio of(B) at least one of the titanium chelate compound, the titanium acylatecompound, the zirconium chelate compound, or the zirconium acylatecompound.

The refractive index of the coating layer can also be increased by theaddition of the metal fine particles. However, if the metal fineparticles are added, the stretchability of the coating layer and theadhesion between the hard coating layer and the base film can bereduced.

In the polyester resin (A) for use in the invention, an active site suchas a hydroxyl group or a carboxyl group may be introduced into itsmolecular chain. Even though such an active site is not introduced, theester bond site can cause a reversible reaction at high temperatures sothat a crosslinking reaction can occur at any site to produce a densefilm.

In order that acrylic resins should have similar crosslinkability, acertain crosslinkable functional group must be introduced. Since therefractive index of acrylic resins themselves is relatively low,however, it would be difficult to control the refractive index to thesame level as that of the coating layer according to the invention, evenif the titanium chelate compound, the titanium acylate compound, thezirconium chelate compound, or the zirconium acylate compound is usedtogether.

In addition, the polyester resin (A), a component of the coating layer,is involved in the adhesion to the base polyester film. Thus, if thecomposition ratio of (A) the aqueous polyester resin to (B) the compound(A/B) is less than 10/90, the adhesion to the base film would bereduced, and the stretchability of the coating layer would also bereduced so that the coating layer cannot be uniform in the stretchingprocess. This can cause a reduction in the transparency necessary foroptical applications and a problem with the adhesion to the hard coatinglayer to be formed on the readily-adhesive layer. If the compositionratio of (A) the aqueous polyester resin to (B) the compound (A/B) ismore than 95/5, the crosslinking with (B) the water-soluble titaniumacylate compound, the water-soluble zirconium chelate compound or thewater-soluble zirconium acylate compound would be poor, and therefractive index would be reduced. This can reduce the adhesion at hightemperatures and high humidities (the resistance to humidity and heat)and make the rainbow reflection-suppressing effect under fluorescentlamps insufficient.

In the invention, the aqueous polyester resin (A) refers to anypolyester resin capable of being dissolved or dispersed in water or anywater-soluble organic solvent (such as an aqueous solution containingless than 50% by mass of an alcohol, alkylcellosolve, ketone or ethersolvent). In order to impart the aqueous properties to any polyesterresin, it is important to introduce a hydrophilic group such as ahydroxyl group, a carboxyl group, a sulfonate group, a phosphate group,and an ether group into the molecular chain of the polyester resin.Among these hydrophilic groups, the sulfonate group is preferred in viewof coating film properties and adhesion.

In a case where the sulfonate group is introduced into polyester, thesulfonic acid compound is more preferably from 1 to 10% by mole based onthe total amount of all the acid components of the polyester. If theamount of the sulfonate group is less than 1% by mole, the polyesterresin would not exhibit the aqueous properties, and it can have reducedcompatibility with (B) at least one of the water-soluble titaniumchelate compound, the water-soluble titanium acylate compound, thewater-soluble zirconium chelate compound, or the water-soluble zirconiumacylate compound, so that it can be difficult to obtain a uniform andtransparent coating layer. If the amount of the sulfonate group is morethan 10% by mole, the adhesion at high temperatures and high humidities(resistance to humidity and heat) can tend to be degraded.

The aqueous polyester resin (A) preferably has a glass transitiontemperature of at least 40° C. For this purpose, an aromatic acid suchas terephthalic acid, isophthalic acid, and naphthalenedicarboxylic acidis preferably used as the main acid component of the polyester resin(A). The glycol component is preferably a glycol with a relatively smallnumber of carbon atoms, such as ethylene glycol, propane glycol,1,4-butanediol, and neopentylglycol, or an aromatic glycol such as anethylene oxide adduct of-bisphenol A. In addition, the materials for thepolyester resin (A) may include a rigid component such as biphenyl or adicarboxylic acid or diol component having a high refractive index atomsuch as bromine and sulfur, as long as the physical properties of thefilm are not degraded. If the glass transition temperature of thepolyester resin (A) is less than 40° C., the adhesion at hightemperatures and high humidities (resistance to humidity and heat) cantend to be insufficient, and the refractive index of the polyester resin(A) can be reduced so that the coating layer can also have a reducedrefractive index. As a result, rainbow reflections can tend to beinsufficiently suppressed under fluorescent lamps.

Another main component for the coating layer is (B) at least one of awater-soluble titanium chelate compound, a water-soluble titaniumacylate compound, a water-soluble zirconium chelate compound, or awater-soluble zirconium acylate compound. The term “water-soluble” meansthat the compound can be dissolved in water or an aqueous solutioncontaining less than 50% by mass of a water-soluble organic solvent.

Examples of the water-soluble titanium chelate compound includediisopropoxybis(acetylacetonato)titanium,isopropoxy(2-ethyl-1,3-hexanediolato)titanium,diisopropoxybis(triethanolaminato)titanium,di-n-butoxybis(triethanolaminat)titanium, hydroxybis(lactato)titanium,ammonium salt of hydroxybis(lactato)titanium, and titanium peroxycitricacid ammonium.

Examples of the water-soluble titanium acylate compound includeoxotitaniumbis(monoammonium oxalate) and the like. Examples of thewater-soluble zirconium compound include zirconium tetraacetylacetonateand zirconium acetate.

In combination with the above main component, the coating layer may alsoinclude any resin other than the above, such as an acrylic resin, apolyurethane resin, a polyester resin, an alkyd resin, and a vinyl resinsuch as polyvinyl alcohol, as long as the advantageous effect of theinvention is not affected. Any crosslinking agent may also be usedtogether, as long as the advantageous effect of the invention is notaffected. Examples of the crosslinking agent for use include adducts offormaldehyde with urea, melamine, benzoguanamine, or the like; aminoresins such as alkyl ether compounds produced from any of these adductsand an alcohol of 1 to 6 carbon atoms; polyfunctional epoxy compounds;polyfunctional isocyanate compounds; blocked isocyanate compounds;polyfunctional aziridine compounds; and oxazoline compounds.

In the invention, the coating liquid for use in forming the coatinglayer is a aqueous coating liquid mainly composed of (A) the aqueouspolyester resin, (B) at least one of the water-soluble titanium chelatecompound, the water-soluble titanium acylate compound, the water-solublezirconium chelate compound, or the water-soluble zirconium acylatecompound, and an water-soluble solvent. For the application of theaqueous coating liquid to the surface of the polyester film, anappropriate amount of any known anionic or nonionic surfactant ispreferably added to the coating liquid such that the wettability to thefilm can be improved for uniform application of the coating liquid.

In order to impart handling properties, antistatic properties,antibiotic properties, or any other functionality to the film, theaqueous coating liquid may also contain an additive such as inorganicand/or heat-resistant polymer particles, an antistatic agent, anultraviolet absorbing agent, an organic lubricant, an antimicrobe agent,and a photo-oxidation catalyst.

Besides water, the solvent for use in the coating liquid may contain analcohol such as ethanol, isopropyl alcohol, and benzyl alcohol, in anamount of less than 50% by mass, based on the total amount. Any organicsolvent other than alcohols may also be included in an amount of lessthan 10% by mass, as long as it can be dissolved. However, the totalamount of the alcohols and the other organic solvents should preferablybe less than 50% by mass.

According to the invention, the laminated polyester film for opticalapplications can be produced by forming a hard coating layer on thecoating layer on at least one side of the readily-adhesive polyesterfilm, wherein the hard coating layer is made from an electronbeam-curable or ultraviolet light-curable acrylic resin or aheat-curable (thermosetting) siloxane resin.

The electron beam-curable or ultraviolet light-curable resin has anacrylate type functional group and for example, may include a relativelylow molecular weight polyester resin, polyether resin, acrylic resin,epoxy resin, urethane resin, alkyd resin, spiroacetal rein, polybutadienresin, or polythiol-polyene resin, or an oligomer or prepolymer of(meth)acrylate or the like of a polyfunctional compound such as apolyhydric alcohol, and a reactive diluent of a monofunctional monomersuch as ethyl (meth)acrylate, ethyl hexyl (meth)acrylate, styrene,methylstyrene, and N-vinylpyrrolidone, or a polyfunctional monomer suchas trimethylolpropane tri(meth)acrylate, hexanediol (meth)acrylate,tripropylene glycol di(meth)acrylate, diethylene glycoldi(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritolhexa(meth)acrylate, 1,6-hexanediol di(meth)acrylte, and neopentyl glycoldi(meth)acrylate.

The ultraviolet light-curable resin may be used in combination with aphotopolymerization initiator such as acetophenones, benzophenones,Michler's benzoylbenzoate, α-amyloxime ester, tetramethylthiurammonosulfide, and thioxanthones, or a photosensitizer such asn-butylamine, triethylamine, and tri-n-butyl phosphine.

Preparation of the Readily-Adhesive Polyester Film

A method for producing the readily-adhesive polyester film of theinvention will be described with reference to an example usingpolyethylene terephthalate (hereinafter abbreviated as PET), but it willbe understood that such a description is not intended to limit theinvention.

A PET resin is sufficiently dried under vacuum and then supplied to anextruder. The PET resin is melted and extruded through a T die at about280° C. into a sheet and fed to a rotary cooling roll. The sheet isallowed to cool and solidified by static electricity impression methodso that an unstretched PET sheet is obtained. The unstretched PET sheetmay have a monolayer structure or a multilayer structure produced bycoextrusion method. In a preferred mode, the PET resin is substantiallyfree of inert particles.

The resulting unstretched PET sheet is stretched 2.5 to 5.0 times in thelongitudinal direction with a roll heated to a temperature of 80 to 120°C. so that a uniaxially-stretched PET film is obtained. The film is thenheld at its end with a clip and introduced into a hot air zone heated toa temperature of 70 to 140° C. and stretched 2.5 to 5.0 times in thewidth direction. The film is subsequently introduced into aheat-treatment zone controlled to temperature of 160 to 240° C. andheat-treated for 1 to 60 seconds result to complete crystallineorientation.

In this film production process, the aqueous coating liquid is appliedto at least one side of the PET film at any stage. The coating layer maybe formed on both sides of the PET film. The concentration of the resincomposition solids in the aqueous coating liquid is preferably from 2 to35% by mass, particularly preferably from 4 to 15% by mass.

Any known method may be used to apply the aqueous coating liquid to thePET film, for example, including reverse roll coating, gravure coating,kiss coating, die coater method, roll brush method, spray coating, airknife coating, wire-bar coating, pipe doctor method, impregnationcoating, and curtain coating. These methods may be used either singly orin combination for the application.

In the invention, it is important to form the coating layer by a processincluding the steps of applying the aqueous coating liquid to theunstretched or uniaxially-stretched PET film, drying the coating andthen stretching the coating in at least one direction and thenperforming heat treatment to the coating. The film coated with thecoating liquid is introduced into a tenter and heated for lateralstretching and heat treatment. In this process, the chelate or acylatecompound can form a stable crosslinked coating layer by a thermalcrosslinking reaction. In contrast, when the coating liquid is appliedto a biaxially-stretched PET film and dried to form a coating layer, thequantity of heat has to be suppressed enough to reduce theheat-treatment-induced degradation of the transparency of the base filmor to reduce the heat-treatment-induced variation in the physicalproperties, so that the amount of heat for the thermal crosslinkingreaction can be too short to form a uniform crosslinked coating layer.

In the invention, the final resulting coating layer preferably has acoating amount of 0.02 to 0.5 g/m². If the coating amount of the coatinglayer is less than 0.02 g/m², there can be little effect on theadhesion, and the rainbow reflection-suppressing effect underfluorescent lamps can also be insufficient. If the coating amount ismore than 0.5 g/m², the rainbow reflection-suppressing effect underfluorescent lamps can also tend to be insufficient.

The coating layer of the readily-adhesive polyester film producedaccording to the invention not only has excellent adhesion to the hardcoating layer made from an electron beam-curable or ultravioletlight-curable acrylic resin or a thermosetting siloxane resin but alsocan exhibit excellent adhesive strength to coatings for applicationsother than optical applications. Examples of such coatings includephotosensitive layers, diazo sensitized layers, mat layers, magneticlayers, inkjet ink-receiving layers, hard coating layers, UV curelayers, thermoset resins, printer's ink, UV ink, adhesives for drylamination, extrusion lamination or the like, thin film layers formed byvacuum deposition, electron beam deposition, sputtering, ion plating,CVD, or the like of metal, inorganic materials or any oxides thereof,and organic barrier layers formed by plasma polymerization.

Production of Laminated Polyester Film for Optical Applications

A method for producing the laminated polyester film for opticalapplications according to the invention will be described with referenceto an example using a PET film, but it will be understood that such adescription is not intended to limit the invention.

A hard-coating-layer-forming coating liquid containing the electronbeam-curable or ultraviolet light-curable acrylic resin or thethermosetting siloxane resin is applied to the coating layer on at leastone side of the readily-adhesive polyester film. The coating liquidshould not always be diluted but may be diluted with an organic solventdepending on the viscosity or wettability of the coating liquid, or thethickness of the coating layer, or the like. The hard coating layer isformed by a process including the steps of applying thehard-coating-layer-forming coating liquid onto the coating layer on atleast one side of the readily-adhesive polyester film, then optionallydrying the coating, and applying an electron beam or ultraviolet lightto the curable resin or heating the curable resin, depending on thecuring conditions, to cure the coating.

In the invention, the hard coating layer preferably has a thickness of 1to 15 μm. A hard coating layer with a thickness of less than 1 μm canhave little effect with respect to chemical resistance, scratchresistance, anti-fouling, or the like. A hard coating layer with athickness of more than 15 μm can have reduced flexibility and anincreased risk of cracking or the like.

The optical laminated polyester film obtained according to the inventioncan find wide applications and may be particularly formed into aexcellent antireflection film by further forming an antireflection layeron the hard coating layer. Such an antireflection layer may be made of amonolayer or multi-layers of an inorganic material(s) such as a highrefractive index material,e.g. ZnO, TiO₂, CeO₂, SnO₂, and ZrO₂, and alow refractive index material, e.g. MgF₂ and SiO₂ or a metalmaterial(s). In order to form such a layer or layers, a monolayer ormulti-layers may be formed by vapor deposition, sputtering, plasma CVDor the like of the high or low refractive index inorganic material(s) orthe metal material(s) or by applying a resin composition containing thehigh or low refractive index inorganic material(s) or the metalmaterial(s).

EXAMPLES

The invention is described in detail using the examples and comparativeexamples below, but it will be understood that the examples are notintended to limit the scope of the invention. The evaluation methodsbelow were used for the invention.

(1) Total Light Transmittance

According to JIS K 7105, a turbidimeter (NDH 2000, manufactured byNippon Denshoku Industries Co., Ltd.) was used to determine the totallight transmittance of the film.

(2) Glass Transition Temperature

According to JIS K 7121, a differential scanning calorimeter (DSC 6200,manufactured by Seiko Instruments Inc.) was used. The temperature wasraised at a rate of 20° C./minute in the range of 25 to 300° C., and anextrapolated glass transition initiation temperature was obtained fromthe DSC curve and defined as the glass transition temperature.

(3) Adhesion

The test method according to Section 8.5.1 of JIS K 5400 is used todetermine the adhesion between the hard coating layer and the base filmin the hard-coated film.

More specifically, using a cutter guide with a slit space of 2 mm, cutsin a grid pattern of 100 squares are made from the surface of the hardcoating layer so as to reach the base film through the hard coatinglayer. A pressure-sensitive adhesive cellophane tape (No. 405, 24 mm inwidth, manufactured by Nichiban Co., Ltd.) is then attached to the cutsurface and completely bonded by rubbing with an eraser. Thepressure-sensitive adhesive cellophane tape is then perpendicularlypeeled from the surface of the hard coating layer of the hard-coatedfilm, and the number of the squares stripped from the hard coating layerside of the hard-coated film is determined by visual counting. Theadhesion between the hard coating layer and the base film is calculatedusing the formula below. Partially stripped squares are also counted asbeing stripped.Adhesion (%)=(1-the number of stripped squares/100)×100(4) Resistance to Humidity and Heat

Hard-coated film was allowed to stand under a circumstance controlled at60° C. and 95RH % for 500 hours in a high-temperature, high-humidityvessel and then removed and allowed to stand at room temperature for 12hours.

Thereafter, the adhesion between the hard coating layer and the basefilm was determined using the method of the above item (3) and rankedaccording to the following criteria.

-   ∘∘: 100%-   ∘: at least 96% and less than 100%-   ▴: at least 80% and less than 96%-   x: less than 80%    (5) Improvement Against Interference Fringes (Rainbow Reflections)

Hard-coated film was cut into a sample film with an area of 10 cm (inthe film width direction) by 15 cm (in the film length direction). Aglossy black tape (Vinyl Tape No. 21, black, manufactured by NITTO DENKOCORPORATION) was bonded to the side opposite to the hard coating layerside of the resulting sample film. The sample film with its hard coatingsurface facing upward was observed under a three-wavelength light sourcewith daylight white color (National Palook FL 15EX-N 15W) in apositional relation where the reflection visually observed fromobliquely above was most intense (with a distance of 40 to 60 cm fromthe light source at an angle of 15 to 45°).

The results of the visual observation are ranked according to thecriteria below. The observation is performed by five members familiarwith this evaluation, and the most frequently selected rank is used forevaluation. If the numbers of the members selecting two ranks,respectively, are the same, the median of the corresponding three ranksis employed, for example, as follows. When ranks ∘∘ and ∘ are eachselected by two members and rank A is selected by one member, rank ∘ isemployed. When rank ∘∘ is selected by one member, and ranks ∘ and ▴ areeach selected by two members, rank ∘ is employed. When ranks ∘∘ and ▴are each selected by two members and rank ∘ is selected by one member,rank ∘ is employed.

-   ∘∘: No rainbow reflection is observed at any angle.-   ∘: Rainbow reflection is slightly observed only at a certain angle.-   ▴: Rainbow reflection is slightly observed.-   x: Rainbow reflection is clearly observed.    Polymerization for Polyester Resin

To a stainless steel autoclave equipped with a stirrer, a thermometerand a partial reflux condenser were added 186 parts by mass of dimethylterephthalate, 186 parts by mass of dimethyl isophthalate, 23.7 parts bymass of dimethyl 5-sodium sulfoisophthalate, 137 parts by mass ofneopentyl glycol, 191 parts by mass of ethylene glycol, and 0.5 parts bymass of tetra-n-butyl titanate and subjected to transesterification attemperatures from 160° C. to 220° C. for 4 hours. The reaction systemwas then heated to 255° C., gradually decompressed and then allowed toreact under a reduced pressure of 29 Pa for one hour and 30 minutes togive a copolyester resin (named A-1). The resulting copolyester resinwas light yellow and transparent.

Polyester resins with different compositions (named A-2, A-3 and A-4,respectively) were obtained using the same process. The compositionsdetermined by NMR and weight average molecular weights of thesecopolyester resins, and the evaluation results are shown in Table 1.TABLE 1 Compositions of Copolymers (% by mole) A-1 A-2 A-3 A-4Dicarboxylic Terephthalic Acid 80 47 85 85 Acid Adipic Acid — — 10 —Components Isophthalic Acid 15 47 — 6 5-Sodium 5 2.5 5 9Sulfoisophthalic Acid Glycol Ethylene Glycol 85 70 70 85 ComponentsNeopentyl Glycol 15 — — 15 1,4-Butanediol — — 30 — Ethylene Oxide — 30 —— Adduct of Bisphenol A Physical Weight Average 55000 53000 60000 8000Properties Molecular Weight Glass Transition 65 75 45 63 Temperature (°C.)

Example 1 (1) Preparation of Aqueous Dispersions of Polyesters

To a reactor equipped with a stirrer, a thermometer and a refluxcondenser were added 20 parts by mass of the polyester resin (A-1) and15 parts by mass of ethylene glycol monobutyl ether and heated andstirred at 100° C. until the resin was dissolved. After the resin wascompletely dissolved, 65 parts by mass of water was gradually added tothe polyester solution with stirring. After the addition, the solutionwas cooled to room temperature with stirring to form an aqueous milkywhite dispersion (named B-1) of the polyester with a solids content of20% by mass. Similarly, aqueous dispersions named (B-2) to (B-4),respectively, were prepared using the polyester resins (A-2) to (A-4),respectively, in place of the polyester resin(A-1).

(2) Preparation of Coating Liquid

40 parts by mass of the resulting aqueous dispersion (B-1) of thepolyester, 18 parts by mass of a 44% by mass hydroxybis(lactato)titaniumsolution (TC310, manufactured by Matsumoto Chemical Industry Co., Ltd.),150 parts by mass of water, and 100 parts by mass of isopropyl alcoholwere mixed, and 1% by mass (based on the amount of a coating liquid) ofan anionic surfactant (Neopelex No. 6F powder, manufactured by KaoCorporation) and 2% by mass (in terms of silica content based on theamount of the resin solids) of an aqueous dispersion of colloidal silicafine particles (Cataloid SI8OP, with an average particle size of 80 nm,manufactured by Catalysts & Chemicals Industries Co., Ltd.) were eachadded to the mixture to form a coating liquid (hereinafter referred toas coating liquid (C-1)).

(3) Preparation of Readily-Adhesive Polyester Film Having Coating Layer

PET resin pellets having an intrinsic viscosity of 0.62 dl/g and beingsubstantially free of particles were provided as a raw polymer materialof film and dried under a reduced pressure of 133 Pa at 135° C. for 6hours. The pellets were then fed to a twin-screw extruder and melted andextruded at about 280° C. into a sheet, which was rapidly allowed tocool and solidify by contacting on a rotary cooling metal roll with itssurface temperature maintained at 20° C. according to a staticelectricity impression method to give an unstretched PET sheet with athickness of 1400 μm.

The unstretched PET sheet was heated to 100° C. with a series of heatedrolls and an infrared heater and then stretched 3.5 times in thelongitudinal direction with a series of rolls with different peripheralspeeds to give a uniaxially-stretched PET film.

The coating liquid (C-1) was then applied to one side of the PET film bya reverse roll method so as to provide a dry coating amount of 0.5 g/m²and then dried at 80° C. for 20 seconds. After the drying, the film wassubsequently stretched 4.0 times in the width direction at 120° C. byusing tenter and heated at 230° C. for 0.5 seconds with its length inthe width direction fixed and then relaxed 3% in the width direction at230° C. for 10 seconds, resulting in a 100 μm-thick biaxially-stretchedPET film having the coating layer on one side.

(4) Preparation of Hard-Coated Film

A solution prepared by adding 5 parts by mass of methyl ethyl ketone to5 parts by mass of a hard coating agent (Seikabeam EXFO1 (B), with asolids content of 100% by mass, manufactured by Dainichiseika Color &Chemicals Mfg. Co., Ltd.) was applied to the coating surface of theresulting readily-adhesive polyester film with a wire bar No. 8 anddried at 70° C. for 1 minute so that the solvent was removed.Ultraviolet light was irradiated by a high pressure mercury lamp to thesurface of the hard coating layer under the conditions of an irradiationenergy of 200 mJ/cm² and a radiation distance of 15 cm, while the filmwith the hard coating layer is fed at a feed rate of 5 m/minute, so thata hard-coated film with a 3 μm-thick hard coating layer was obtained.The evaluation results are shown in Table 2.

Example 2

48 parts by mass of the aqueous dispersion (B-2) of the polyester, 15parts by mass of a 44% by mass hydroxybis(lactato)titanium solution(TC310, manufactured by Matsumoto Chemical Industry Co., Ltd.), 150parts by mass of water, and 100 parts by mass of isopropyl alcohol weremixed, and 1% by mass (based on the amount of a coating liquid) of ananionic surfactant (Neopelex No. 6F powder, manufactured by KaoCorporation) and 2% by mass (in terms of silica content based on theamount of the resin solids) of an aqueous dispersion of colloidal silicafine particles (Cataloid SI80P, with an average particle size of 80 nm,manufactured by Catalysts & Chemicals Industries Co, Ltd.) were eachadded to the mixture to form a coating liquid (hereinafter referred toas coating liquid (C-2)). A biaxially-stretched PET film having acoating layer on one side and a hard-coated film were obtained using thecoating liquid and the process of Example 1. The evaluation results areshown in Table 2.

Example 3

12 parts by mass of the aqueous dispersion (B-3) of the 25 polyester, 17parts by mass of an 80% by massdiisopropoxybis(triethanolaminato)titanium solution (TC400, manufacturedby Matsumoto Chemical Industry Co., Ltd.), 150 parts by mass of water,and 100 parts by mass of isopropyl alcohol were mixed, and 1% by mass(based on the amount of a coating 30 liquid) of an anionic surfactant(Neopelex No. 6F powder, manufactured by Kao Corporation) and 2% by mass(in terms of silica content based on the amount of the resin solids) ofan aqueous dispersion of colloidal silica fine particles (CataloidSI8OP, with an average particle size of 80 nm, manufactured by Catalysts& Chemicals Industries Co, Ltd.) were each added to the mixture to forma coating liquid (hereinafter referred to as coating liquid (C-3)). Abiaxially-stretched PET film having a coating layer on one side and ahard-coated film were obtained using the coating liquid and the processof Example 1. The evaluation results are shown in Table 2.

Example 4

24 parts by mass of the aqueous dispersion (B-4) of the polyester, 11parts by mass of diisopropoxybis(acetylacetonato)titanium, 150 parts bymass of water, and 100 parts by mass of isopropyl alcohol were mixed,and 1% by mass (based on the amount of a coating liquid) of an anionicsurfactant (Neopelex No. 6F powder, manufactured by Kao Corporation) and2% by mass (in terms of silica content based on the amount of the resinsolids) of an aqueous dispersion of colloidal silica fine particles(Cataloid SI8OP, with an average particle size of 80 nm, manufactured byCatalysts & Chemicals Industries Co, Ltd.) were each added to themixture to form a coating liquid (hereinafter referred to as coatingliquid (C-4)). A biaxially-stretched PET film having a coating layer onone side and a hard-coated film were obtained using the coating liquidand the process of Example 1. The evaluation results are shown in Table2.

Example 5

32 parts by mass of the aqueous dispersion (B-4) of the polyester, 10parts by mass of zirconium acetate, 150 parts by mass of water, and 100parts by mass of isopropyl alcohol were mixed, and 1% by mass (based onthe amount of a coating liquid) of an anionic surfactant (Neopelex No.6F powder, manufactured by Kao Corporation) and 2% by mass (in terms ofsilica content based on the amount of the resin solids) of an aqueousdispersion of colloidal silica fine particles (Cataloid SI80P, with anaverage particle size of 80 nm, manufactured by Catalysts & ChemicalsIndustries Co, Ltd.) were each added to the mixture to form a coatingliquid (hereinafter referred to as coating liquid (C-5)). Abiaxially-stretched PET film having a coating layer on one side and ahard-coated film were obtained using the coating liquid and the processof Example 1. The evaluation results are shown in Table 2.

Comparative Example 1

80 parts by mass of the aqueous dispersion (B-1) of the polyester, 150parts by mass of water, and 100 parts by mass of isopropyl alcohol weremixed, and 1% by mass (based on the amount of a coating liquid) of ananionic surfactant (Neopelex No. 6F powder, manufactured by KaoCorporation) and 2% by mass (in terms of silica content based on theamount of the resin solids) of an aqueous dispersion of colloidal silicafine particles (Cataloid SI80P, with an average particle size of 80 nm,manufactured by Catalysts & Chemicals Industries Co, Ltd.) were eachadded to the mixture to form a coating liquid (hereinafter referred toas coating liquid (C-6)). A biaxially-stretched PET film having acoating layer on one side and a hard-coated film were obtained using thecoating liquid and the process of Example 1. The evaluation results areshown in Table 2.

Comparative Example 2

64 parts by mass of the aqueous dispersion (B-1) of the polyester, 10parts by mass of a self cross-linkable polyurethane resin having ablocked isocyanate group (Elastron H-3, manufactured by Dai-ichi KogyoSeiyaku Co., Ltd.), and 1 part by mass of a catalyst for Elastron(Cat64, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) were mixed,and 1% by mass (based on the amount of a coating liquid) of an anionicsurfactant (Neopelex No. 6F powder, manufactured by Kao Corporation) and2% by mass (in terms of silica content based on the amount of the resinsolids) of an aqueous dispersion of colloidal silica fine particles(Cataloid SI8OP, with an average particle size of 80 nm, manufactured byCatalysts & Chemicals Industries Co, Ltd.) were each added to themixture to form a coating liquid (hereinafter referred to as coatingliquid (C-7)). A biaxially-stretched PET film having a coating layer onone side and a hard-coated film were obtained using the coating liquidand the process of Example 1. The evaluation results are shown in Table2.

Comparative Example 3

40 parts by mass of a 44% by mass hydroxybis(lactato)titanium solution(TC310, manufactured by Matsumoto Chemical Industry Co., Ltd.), 150parts by mass of water, and 100 parts by mass of isopropyl alcohol weremixed, and 1% by mass (based on the amount of a coating liquid) of ananionic surfactant (Neopelex No. 6F powder, manufactured by KaoCorporation) and 2% by mass (in terms of silica content based on theamount of the resin solids) of an aqueous dispersion of colloidal silicafine particles (Cataloid SI8OP, with an average particle size of 80 nm,manufactured by Catalysts & Chemicals Industries Co, Ltd.) were eachadded to the mixture to form a coating liquid (hereinafter referred toas coating liquid (C-8)). A biaxially-stretched PET film having acoating layer on one side and a hard-coated film were obtained using thecoating liquid and the process of Example 1. The evaluation results areshown in Table 2.

Comparative Example 4

32 parts by mass of the aqueous dispersion (B-2) of the polyester, 5parts by mass of a self cross-linkable polyurethane resin having ablocked isocyanate group (Elastron H-3, manufactured by Dai-ichi KogyoSeiyaku Co., Ltd.), 0.5 parts by mass of a catalyst for Elastron (Cat64,manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.), and 64 parts by massof an aqueous 10% by mass niobium oxide sol solution (SAM-O,manufactured by Taki Chemical Co., Ltd.) were mixed, and 1% by mass(based on the amount of a coating liquid) of an anionic surfactant(Neopelex No. 6F powder, manufactured by Kao Corporation) and 2% by mass(in terms of silica content based on the amount of the resin solids) ofan aqueous dispersion of colloidal silica fine particles (CataloidSI8OP, with an average particle size of 80 nm, manufactured by Catalysts& Chemicals Industries Co, Ltd.) were each added to the mixture to forma coating liquid (hereinafter referred to as coating liquid (C-9)). Abiaxially-stretched PET film having a coating layer on one side and ahard-coated film were obtained using the coating liquid and the processof Example 1. The evaluation results are shown in Table 2.

Comparative Example 5

80 parts by mass of an emulsion of an acrylic resin (methylmethacrylate/ethyl acrylate/acrylic acid/N-methylolacrylamide=60/40/2/4in mass ratio) with a solids content of 20% by mass, 3.2 parts by massof di-n-butoxybis(triethanolaminato)titanium, 150 parts by mass ofwater, and 100 parts by mass of isopropyl alcohol were mixed, and 1% bymass (based on the amount of a coating liquid) of an anionic surfactant(Neopelex No. 6F powder, manufactured by Kao Corporation) and 2% by mass(in terms of silica content based on the amount of the resin solids) ofan aqueous dispersion of colloidal silica fine particles (CataloidSI8OP, with an average particle size of 80 nm, manufactured by Catalysts& Chemicals Industries Co, Ltd.) were each added to the mixture to forma coating liquid (hereinafter referred to as coating liquid (C-10)). Abiaxially-stretched PET film having a coating layer on one side and ahard-coated film were obtained using the coating liquid and the processof Example 1. The evaluation results are shown in Table 2.

Comparative Example 6

Forty-eight parts by mass of an emulsion of an acrylic resin (methylmethacrylate/ethyl acrylate/acrylic acid/N-methylolacrylamide=25/75/4/2in mass ratio) with a solids content of 20% by mass, 6.4 parts by massof an aqueous titanium-modified resin (Orgatix WS680, manufactured byMatsumoto Chemical Industry Co., Ltd.), 150 parts by mass of water, and100 parts by mass of isopropyl alcohol were mixed, and 1% by mass (basedon the amount of a coating liquid) of an anionic surfactant (NeopelexNo. 6F powder, manufactured by Kao Corporation) and 2% by mass (in termsof silica content based on the amount of the resin solids) of an aqueousdispersion of colloidal silica fine particles (Cataloid SI8OP, with anaverage particle size of 80 nm, manufactured by Catalysts & ChemicalsIndustries Co, Ltd.) were each added to the mixture to form a coatingliquid (hereinafter referred to as coating liquid (C-11)). Abiaxially-stretched PET film having a coating layer on one side and ahard-coated film were obtained using the coating liquid and the processof Example 1. The evaluation results are shown in Table 2.

Comparative Example 7

An uncoated biaxially-stretched PET film was obtained using the processof Example 1 except that no readily-adhesive-layer-forming coatingliquid was used. A hard coating layer was formed on one side of theuncoated film using the process of Example 1 so that a hard-coated filmwas prepared. The evaluation results are shown in Table 2.

Comparative Example 8

10 parts by mass of titanium oxide ultra-fine particles (TTO-S-1,manufactured by Ishihara Sangyo Kaisha, Ltd.) with particle sizes(width/length) of 0.01 to 0.02 μm/0.05 to 0.1 μm observed with anelectron microscope was mixed with 90 parts by mass of water anddispersed at 5000 rpm for 30 minutes in a dispersing machine (AUTO CELLMASTER CM-200) so that an aqueous dispersion of the titanium oxideparticles at a concentration of 10% by mass (named aqueous dispersion A)was prepared.

Thirty parts by mass of the aqueous dispersion (B-4) of the polyester,150 parts by mass of water, and 100 parts by mass of isopropyl alcoholwere then mixed, and 1% by mass (based on the amount of a coatingliquid) of an anionic surfactant (Neopelex No. 6F powder, manufacturedby Kao Corporation) was added to the mixture to form an aqueousdispersion of the polyester (named aqueous dispersion B). Thirty partsby mass of the aqueous dispersion A of the titanium oxide particlesprepared as described above was added to the aqueous dispersion B of thepolyester to form a coating liquid. In the coating liquid, however, thetitanium oxide fine particles were precipitated in the form of gel, andthus coating the base film was stopped. TABLE 2 Components of ResinComposition for Coating Layer Characteristic Values Component Resistanceto Additional Ratio Total Light Adhesion Humidity and Interference ResinA Resin Compound B (A)/(B) Transmittance (%) (%) Heat Fringe Example 1A-1 — Hydroxybis(lactato) 50.3/49.7 90 100 ◯◯ ◯◯ titanium Example 2 A-2— Hydroxybis(lactato) 59.3/40.7 89 100 ◯◯ ◯◯ titanium Example 3 A-3 —Diisopropoxybis 15.0/85.0 90 100 ◯◯ ◯◯ (triethanolaminato) titaniumExample 4 A-4 — Diisopropoxybis 30.4/69.6 91 100 ◯◯ ◯◯ (acetylacetonato)titanium Example 5 A-4 — Zirconium Acetate 39.0/61.0 90 100 ◯◯ ◯◯Comparative A-1 — — — 90 90 ▴ X Example 1 Comparative A-1 Crosslinkable— — 91 100 ◯ X Example 2 Polyurethane Comparative — —Hydroxybis(lactato) — 83 20 — ▴ Example 3 titanium Comparative A-2Crosslinkable — — 89 80 X ◯ Example 4 Polyurethane Comparative — AcrylicResin Di-n-butoxybis — 90 100 ◯ X Example 5 (triethanolaminato)titaniumComparative — Titanium- — — 90 100 ◯ X Example 6 Modified Acrylic ResinComparative — — — — 92 0 — X Example 7 Comparative A-4 — — — — — — —Example 8

INDUSTRIAL APPLICABILITY

When a hard coating layer is laminated on the readily-adhesive layer ofthe optical readily-adhesive polyester film of the invention, the filmof the invention demonstrates excellent anti-reflective properties tosuppress the reflection of external light, glare, rainbow reflections,and the like and exhibits excellent adhesion to the hard coating layerand exhibits excellent adhesion at high temperatures and high humidities(resistance to humidity and heat). Thus, the film of the invention isuseful as a base material film for antireflection films which areattached to the front side of a display screen of a tough panel, aliquid crystal display (LCD), a cathode-ray-tube (CRT) for a televisionset or computer, a plasma display (PDP), a decoration, or the like toimpart antireflection properties such that the reflection of externallight, glare, rainbow reflections, and the like can be suppressed.Additionally, the film of the invention exhibits excellent adhesionbetween the readily-adhesive layer and the layer placed thereon andexhibits excellent adhesion at high temperatures and high humidities(resistance to humidity and heat). Thus, the layer placed on thereadily-adhesive layer may be not only the hard coating layer foroptical applications but also any other layer comprising any of a widevariety of materials, such as a photosensitive layer, a diazo sensitizedlayer, a mat layer, an ink layer, an adhesive layer, a thermoset resinlayer, a UV cure resin layer, a vapor-deposited layer of metal orinorganic oxide.

1. A readily-adhesive polyester film for optical applications, comprising: a biaxially-stretched polyester film; and a coating layer that is stacked on at least one side of the polyester film and produced applying, to at least one side of the polyester film, an aqueous coating liquid containing a resin composition comprising (A) an aqueous polyester resin and (B) at least one compound selected from the group consisting of a water-soluble titanium chelate compound, a water-soluble titanium acylate compound, a water-soluble zirconium chelate compound and a water-soluble zirconium acylate compound as main components, wherein the mixing ratio (A)/(B) being from 10/90 to 95/5 by weight; drying the coating; and then stretching the coating in at least one direction.
 2. The readily-adhesive polyester film for optical applications according to claim 1, having a total light transmittance of at least 85%.
 3. The readily-adhesive polyester film for optical applications according to claim 1 or 2, wherein the aqueous polyester resin (A) is a copolyester resin containing 1 to 10% by mole of a metal sulfonate group-containing aromatic dicarboxylic acid component based on the total amount of all the dicarboxylic acid components of the polyester.
 4. The readily-adhesive polyester film for optical applications according to claim 1 or 2, wherein the aqueous polyester resin (A) has a glass transition temperature of at least 40° C.
 5. A laminated polyester film for optical applications, comprising: a readily-adhesive polyester film for optical applications comprising a biaxially-stretched polyester film; and a coating layer that is stacked on at least one side of the polyester film and produced applying, to at least one side of the polyester film, an aqueous coating liquid containing a resin composition comprising (A) an aqueous polyester resin and (B) at least one compound selected from the group consisting of a water-soluble titanium chelate compound, a water-soluble titanium acylate compound, a water-soluble zirconium chelate compound and a water-soluble zirconium acylate compound as main components, the mixing ratio (A)/(B) being from 10/90 to 95/5 by weight; drying the coating; and then stretching the coating in at least one direction; and a hard coating layer that is stacked on the coating layer on at least one side of the readily-adhesive polyester film and comprises an electron beam-cured or ultraviolet light-cured acrylic resin or a heat-cured siloxane resin.
 6. The readily-adhesive polyester film for optical applications according to claim 3, wherein the aqueous polyester resin (A) has a glass transition temperature of at least 40° C.
 7. The laminated adhesive polyester film for optical applications according to claim 5 having a total light transmittance of at least 85%.
 8. The laminated polyester film for optical applications according to claim 5 or 7, wherein the aqueous polyester resin (A) is a copolyester resin containing 1 to 10% by mole of a metal sulfonate group-containing aromatic dicarboxylic acid component based on the total amount of all the dicarboxylic acid components of the polyester.
 9. The laminated polyester film for optical applications according to claim 5 or 7, wherein the aqueous polyester resin (A) has a glass transition temperature of at least 40° C.
 10. The laminated polyester film for optical applications according to claim 8, wherein the aqueous polyester resin (A) has a glass transition temperature of at least 40° C. 